Pattern registration has always been a key part of successful photolithography. During the manufacture of integrated circuits many masks are used in succession and, in almost all cases, any given mask will need to be aligned relative to its predecessors with a degree of precision that is at least as well controlled as other pattern-related features such as line width.
A particular example of critical tolerances during pattern registration can be seen in the pattern format known as box-in-box. Referring now to FIG. 1a, we show an outer box 2 which has already been formed by photolithography. Inside 2 a smaller box 3 must be formed in a separate photolithograpic step. Both boxes are square shaped and the tolerances involved are extremely tight. Typically, the dimension 4 of the outer box would be in the range of between about 10 and 40 microns while that of the inner box would be between about 5 and 30 microns. Thus, the separation between boxes (for example in the region marked 5) is typically between about 5 and 20 microns.
In practice, to ensure perfect registration between masks, four box-in-box alignments are performed simultaneously, the boxes being located at the four corners of the image field. Generally, the outer box is formed from metal level 1 and the inner box from metal level 2.
It is often the case that proper box-in-box registration can present a problem because the outer box is difficult to see. It can be particularly difficult to see if the connectors are based on damascene technology. The term `damascene` is derived from a form of inlaid metal jewelery first seen in the city of Damascus. In the context of integrated circuits it implies a patterned layer imbedded on and in another layer such that the top surfaces of the two layers are coplanar.
This can be better understood by reference to FIG. 1b which is a cross-section of FIG. 1a through 1b--1b. The outer box is seen to be a metal filled trench 12 which extends downwards below the surface of dielectric layer 10. 10 has been overcoated with inter-metal dielectric layer 15. The inner box is a photoresist pattern 18 which will be used to protect portions of layer 15 during etching. During alignment, trench 12 is very difficult to see because of poor contrast.
In FIG. 2a we show a slightly different version of a box-in-box, favored in cases where Damascene is being applied. Inner box 23 is essentially the same as before but the outer box no longer extends over the full area below 23. Instead it takes the form of a hollow square that surrounds the inner box. For ease of reference, we call such a box a ring shaped box. The region between the inner and outer dimensions of the ring shaped box will be referred to as a stripe.
A cross-section of the ring shaped box seen in FIG. 2a is shown in FIG. 2b. The outer box is seen to be made up of the damascene conductor 22. As was the case with the first example, an outer box of this type is often difficult to see during an alignment process.
A number of approaches to simplifying the alignment process can be found in the prior art. For example, Yim et al.(U.S. Pat. No. 5,329,334 Jul. 1994) describes a test reticle that includes a number of orthogonally arranged alignment marks of various shapes and sizes. Bae (U.S. Pat. No. 5,635,336 Jun. 1997) improves the box-in-box procedure by forming a groove along the inside boundary line of the outer box, while Hsia et al. (U.S. Pat. No. 5,701,013 Dec. 1997 describe an alignment pattern that combines the wafer critical dimension and the box-in-box overlay target into a single structure, allowing them both to be checked in a single operation.
It should be noted that none of the above-cited examples of the prior art mention the damascene process or the alignment problems peculiar to that particular process.